Ionomers and methods to produce them, are well known in the art. The present invention relates to those ionomers which are formed by a neutralization or saponification reaction of a copolymer comprising an alpha olefin and a vinyl monomer having a carboxyl group. Such copolymers include interpolymers as well as grafted copolymers and block copolymers. The neutralization reaction is such that metal salts are formed on at least some of the carboxyl groups. Such ionomers can be used in premium applications such as film tie layers, heat seal layers, golf ball covers, and for impact modification. For many of these applications it is desired that the ionomers exhibit high clarity with minimal gels.
U.S. Pat. No. 3,264,272 describes melt blending a solution containing metallic ions with a copolymer comprising an alpha-olefin and an alpha beta ethylenicaly unsaturated carboxylic acid. It is not believed that the melt blending methods disclosed in this reference would produce ionomers of the consistency and clarity desired.
U.S. Pat. No. 4,847,164 describes a process of incorporating a metal oxide into an ethylene copolymer to form a concentrate or masterbatch and then melt blending a minor amount of the concentrate into an ethylene/carboxylic acid copolymer. This process is reported to achieve a more uniform dispersion of the metal oxide, and avoids reported problems of bubbling when aqueous solutions of metal compounds are used.
U.S. Pat. No. 5,631,328 discloses ionomers having a haze of no more than 10 percent as measured by ASTM method D1003. The reference describes a method which comprises contacting a Group IA metal containing solution with a molten or fluid copolymer comprising alpha-olefins having from two to eight carbon atoms and esters of alpha, beta ethylenically-unsaturated carboxylic acids having from four to twenty two carbon atoms and intensively mixing the copolymer and Group IA metal-containing solution at a temperature and to an extent which provides an ionomers composition having no more than 10 percent haze. While ionomers prepared in such a manner are useful, they are still subject to the occurrence of white specs, which detract from the utility of such materials.
The present invention includes a reactive extrusion process for producing a polymeric ionomer comprising the steps of adding a polymer containing a carboxyl functionality to an extruder; homogenously melting the polymer in a melting zone of the extruder; forming a melt seal zone of the extruder; adding an aqueous solution of metallic ions to the molten polymer in an injection zone of the extruder, wherein the injection zone is located after the melt zone of the extruder; mixing the aqueous solution of metallic ions with the polymer containing the carboxyl functionality in a reaction zone of the extruder under conditions such that the metallic ions are substantially reacted with the carboxyl functionality; and then discharging the reacted ionomers. The process is characterized by ensuring that the pressure in the area of the reactive extruder surrounding the injection zone is sufficiently high to ensure that the aqueous solution remains in solution for a sufficient time to allow for the metallic ions to substantially react with the carboxyl functionality. Ideally the melt seal zone is maintained at a pressure higher than the highest vapor pressure of the aqueous solution during the injection zone and the reaction zone, so as to ensure that no water vapor travels against the direction of the polymer flow. The pressure in the injection and reaction zones should be sufficiently high to prevent the water in the aqueous solution from vaporizing until there has been sufficient residence time to allow for a substantially complete reaction of the metallic ions with the carboxyl groups of the polymer. This can optionally be done with the aid of a second melt seal zone after the reaction zone which is maintained at a pressure higher than the highest vapor pressure of the aqueous solution during the injection zone and the reaction zone, although as long as there is a pressure build-up zone which slows the vaporization up enough to allow for a sufficient residence time (given the mixing conditions in the reaction zone) to allow the desired reaction between the metallic ions in solution and the carboxyl functionality in the polymer to be substantially complete, a total seal is not necessary. It should be understood that this pressure build-up zone can be part of the reaction zone itself or be located after the reaction zone as long as it generates sufficient back pressure to ensure that the aqueous solution does not vaporize in the reaction zone.
In another aspect of the present invention, novel ionomers compositions are provided as distinguished by their clarity and the reduced amounts of gels or other inclusions.